The German patent specification DE 35 12 189 [U.S. Pat. No. 4,683,577] discloses a method and a device for regulating electric-arc furnaces. The purpose is to enable precision adjustment of the electric arc voltage and the electrode height in a manner that is economical and technically feasible without great effort. The actuator for transformer voltage is always controlled by a power regulation. The power regulator superimposed on the current regulator also provides the reference variable for the current regulator. In all cases, only the current regulator acts directly on the electrode adjustment. For the tap changer drive used for the transformer, this therefore results in the possibility to either feed the transformer voltage directly via a set-point specification or to adjust it via the tap changer by means of the mentioned power regulator. The lift drive is actuated via a current regulator, with the respective control voltage being supplied either from a current regulator or from a wear regulator or directly as a specified target value.
The European patent application EP 2 362 710 [US 2012/0320942] discloses an electric-arc furnace and a method for operating an electric-arc furnace. The electric arc assigned to the at least one electrode has a first radiant power that results on the basis of a first adjusted set of operating parameters. The electric-arc furnace is operated according to a specified operation program that is based on an expected process sequence. Monitoring is conducted as to whether there is an undesired deviation between the actual process sequence and the expected process sequence. If there is a deviation, a modified second radiant power is specified. By means of the second radiant power, a modified second set of operating parameters is determined. The method allows to achieve an as short as possible smelting duration while protecting the operating means, in particular the electric-arc furnace cooling system.
The German patent application DE 35 43 773 [U.S. Pat. No. 4,689,800] describes a method for operating an electric-arc furnace such that it is possible with fluctuating raw materials to smelt this material at a minimum value of the drawn electrical energy consumption. The furnace transformer is provided with a load switch, thus making it possible to adjust the output voltage at the secondary side of the transformer. The control is carried out by modifying the taps of the furnace transformer or by lifting and lowering the graphite electrodes by an electrode lifting device in order to change the length of the electric arc. At the same time, the electric current flowing from the secondary side of the furnace transformer to the arc electrode is measured.
The German patent application DE 10 2009 017 196 [U.S. Pat. No. 8,624,565] discloses a tap changer with semiconductor switching components for uninterrupted switching between fixed tap changer contacts that are electrically connected with winding taps of a tapped transformer. In this context, each of the fixed tap changer contacts is either directly connectable with a load dissipation or, during switchover, connectable via the interconnected semiconductor switching components. The load dissipation has fixed, divided dissipation contact pieces so that the semiconductor switching components are galvanically isolated from the transformer winding during stationary operation. There are, however, various disadvantages to tap changers with semiconductor switching components. The permanent application of operating voltage and the strain on the power electronics by lightning impulse voltage necessitate large isolation distances, which are not desirable.
The German patent application DE 27 42 221 discloses a method for preventing disruptive flicker occurrences during operation of electric-arc furnaces. The electrical energy is supplied via a transformer with tap changer control, with the flicker level being detected by a flicker-measuring device. In an evaluation device, the measuring results are processed to a signal that is compared with a specified set-point corresponding to the permissible flicker base. On exceeding the set-point over a specified time interval, a downstream control device delivers a control impulse for switching the on-load tap changer to a lower secondary voltage step. On falling below the lower set-point, the control device triggers a higher secondary voltage step.
As known from the prior art, the electrical components for controlling or regulating the operation of an electric-arc furnace are a furnace transformer, a choke coil, and an electrode support arm system. The energy supply for the alternating current electric-arc furnaces is carried out via furnace transformers with an integrated tap changer. The corresponding energy input can be adjusted by the transformer stages.
A choke coil, which is switchable under load and connected upstream of the transformer, serves for regulating the reactance of the current circuit and thus enables operating the furnace with stable electric arcs as well as limiting the short circuit current. The suitable stage is selected both for the transformer and for the series-connected choke in dependence on process progress. This can be effected by manual intervention from the furnace operator, by an integrated control, or by regulation.
In manual control, an experienced furnace operator can assess the process state by the state of the melting material. This is a possibility for subjective observation of the furnace state and the smelting process. The transformer stage is adjusted in critical situations (for instance, damage to the refractory).
In automatic control, the transformer stages and the choke stages, as the case may be, are adapted depending on the present energy input. In order to maintain the electric arc as stable as possible, a high inductance is generally required in the initial “drilling phase” (OLTC choke==highest stage). The series-connected choke is switched off in the last phase “liquid bath” in order to reduce the reactive power.
A lower voltage step (short electric arcs) is selected during the drilling phase to protect the refractory lining of the furnace (the refractory) as well as the furnace lid. After the electric arc has been covered in foaming slag, the highest voltage step is selected to achieve the highest energy input into the melt. To ensure the high energy input during the final phase, a slightly lower step voltage is selected, while using the maximum current setting.
In particular in the manual and automatic control processes, the above mentioned specifications only very inadequately measure up to the actual process state. Even the newest regulations are also not able to react with the appropriate time constants (for example in the range of milliseconds) to the quick changes in the system.
With regard to tap changers in furnace transformers and choke coils and depending on the diverse switching strategies of the customers, the high switching frequencies are regarded as a technical stress factor. This is primarily attributed to contact erosion and to wear of the mechanical components in the tap changers.
Maintenance work on tap changers normally implies a high effort and, above all, cost-intensive production downtime, making it definitely desirable for the operator to extend the maintenance interval in order to reduce the maintenance effort for the tap changer as much as possible.
Furthermore, the frequent switching processes result in additional network feedback, for instance in the form of “flickering” that has to be reduced in a very elaborate and cost-intensive manner (for example SVC systems).